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A hybrid of nanostructured Fe/Tb multilayer ferromagnets and ZnCdMnSe dilute magnetic semiconductor (DMS) quantum well was studied by magneto-micro-photoluminescence (µPL) spectroscopy. When the sample is magnetized in an external magnetic field an additional fringe field from the nanomagnets effectively modulates the local exciton spin states in the quantum well (QW) due to the giant Zeeman effect. Observation of 1.5% of remanent PL polarization degree from the QW gives a direct proof of remanent exciton spin polarization in the DMS. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of the strain on the dot morphology of GaInAs quantum dots has been investigated. The strain was varied by the In content in GaInAs/GaAs quantum dots from 60% down to 30% by keeping the emission wavelength at about 900 nm at 10 K. Spectral properties are compared with morphological results determined by scanning electron and scanning transmission electron microscopy confirming a change of the dot geometry from circular to elongated shapes during an overgrowth process. These lowly strained quantum dot layers with enlarged dot sizes exhibit a reduced dot density of 6–9×10 9 cm −2 and a strongly enhanced oscillator strength, which make them very interesting for single quantum dot and cavity quantum electrodynamic experiments as well as for applications like single photon emitters.

We present an improved method for measuring currents in packaged integrated circuits (IC) for the purpose of test and failure analysis. We use a quartz resonator, called needle sensor, to detect the magnetic field of the device under test (DUT). Thus, the measurement principle is similar to conventional magnetic force microscopy. Compared to a cantilever-based scanning force microscope the advantage of a needle sensor is the much easier test access because of its geometry. With this probe we realized current measurements with a sensitivity of 100 μA. The results are verified by using a simple model describing the basic principle of our measurement setup.

Self-assembled Cd(Mn)Se/Zn(Mn)Se quantum dots have been investigated by means of spatially and time-resolved magneto-optical spectroscopy. In such quasi zero-dimensional diluted magnetic semiconductors, the exchange interaction couples the spins of optically generated charge carriers with localized magnetic ion spins. We demonstrate that this can be used on the one hand to monitor nanoscale magnetization with a resolution of

We investigated InAs-Dots-in-a-well structures emitting near 1.3 μm by bright field and Z-contrast mode in a scanning transmission electron microscope. The chemically sensitive Z-contrast mode is found to give direct information on the actual position of the InAs-Dots inside the embedding well, while the bright field mode monitors the strain fields. Comparing a series of structures, we found that the most symmetric design is realized by an nominally asymmetric growth. These symmetric structures exhibit the best performance with respect to photoluminescence spectra and laser threshold current density.

By combined pulsed auto- and cross-correlation measurements (4K) on individual self-assembled CdSe quantum dots, besides triggered single photon emission promising for high repetition frequencies, in particular the generation of time-correlated photon pairs from the sequential radiative biexciton-exciton decay is demonstrated. For the photon pairs a strong collinear polarization degree up to 84.5% was observed which is related to an exciton fine-structure splitting. Together with the measured exciton lifetime an estimate for the relaxation time (≥480 ps) between the fine-structure levels is given.

We report on the generation of triggered single photons and photon pairs relying on pulsed optical excitation of epitaxially grown self-assembled CdSe quantum dots (QDs). Single-photon emission is studied on individual CdSe/Zn(S,Se) QDs for temperatures up to 200 K. At low temperatures nearly perfect single-photon emission is achieved whereas at higher temperatures an increasing multi-photon emission probability due to spectrally overlapping acoustic phonon sidebands of neighboring QDs is observed. The multi-photon emission probability of a bare QD (background subtracted) is strongly suppressed. Furthermore, the polarization cross correlation of the biexciton-exciton cascade has been investigated in a CdSe/ZnSe QD at low temperatures. A strong polarization correlation of the emitted photon pairs of ∼74% is observed in a linear detection basis.

We report on a new approach for preparing single pairs of quantum dots with controllable interdot coupling. Starting from a double quantum well heterostructure, a selective intermixing technique was applied to define pairs of vertically stacked single quantum dots. The incorporation of Mn 2+ ions in only one of the dots results in strongly different effective g-factors of the dots, enabling us to control the interdot coupling simply by applying an external magnetic field.

The photoluminescene signal of individual semimagnetic CdSe–Zn0.75Mn0.25Se quantum dots is used to study the magnetization of the Mn2+ spin system in the exchange field of a single exciton. We demonstrate that by increasing the laser excitation power a significant blue shift of the photoluminescence signal occurs. This is attributed to a laser-induced demagnetization, i.e. the laser-generated carriers heat the Mn2+ spin system via spin–flip exchange scattering.

We have studied the exciton spin dynamics in CdSe/ZnSe quantum dots, comparing strictly resonant and nonresonant excitation. In case of strictly resonant excitation, excitons are generated in the quantum dot ground state, and because of the zero-dimensionality of the system no transient shift of the photoluminescence signal can be seen. No loss of the spin information is observed within the time window under investigation, if one excites the quantum dot eigenstates. Interestingly, even in case of nonresonant excitation, a high, time-independent polarization degree is obtained. We found maxima in the absolute value of the polarization degree if the laser excess energy amounts to a multiple of LO-phonon energies.

We demonstrate gate control of the carrier distribution in $k$-space in bilayer and monolayer ${\mathrm{MoS}}_{2}$ devices, probed by microphotoluminescence spectroscopy on a contacted single flake. The characteristic emission lines of the neutral and the negatively charged exciton act as a sensor for electron depletion/agglomeration via gate voltage. Gate-induced carrier depletion enhances the sensitivity to defects in monolayers, whereas in bilayers the indirect transition becomes more weight. The specific band structure of bilayers results in a thermal dissociation of trions at 200 K and above, in contrast to monolayers where trion emission is observed up to room temperature. We show that these findings are a consequence of a bias-driven redistribution of charge carriers between the different band minima.

Doping semiconductor nanocrystals with magnetic transition-metal ions has attracted fundamental interest to obtain a nanoscale dilute magnetic semiconductor, which has unique spin exchange interaction between magnetic spin and exciton. So far, the study on the doped semiconductor NCs has usually been conducted with NCs with larger than 2 nm because of synthetic challenges. Herein, we report the synthesis and characterization of Mn2+-doped (CdSe)(13) clusters, the smallest doped semiconductors. In this study, single-sized doped clusters are produced in large scale. Despite their small size, these clusters have semiconductor band structure instead of that of molecules. Surprisingly, the clusters show multiple excitonic transitions with different magneto-optical activities, which can be attributed to the fine structure splitting. Magneto-optically active states exhibit giant Zeeman splittings up to elevated temperatures (128 K) with large g-factors of 81(+/- 8) at 4 K. Our results present a new synthetic method for doped clusters and facilitate the understanding of doped semiconductor at the boundary of molecules and quantum nanostructure. Refereed/Peer-reviewed

The high speed on–off performance of GaN-based light-emitting diodes (LEDs) grown in c-plane direction is limited by long carrier lifetimes caused by spontaneous and piezoelectric polarization. This work demonstrates that this limitation can be overcome by m-planar core–shell InGaN/GaN nanowire LEDs grown on Si(111). Time-resolved electroluminescence studies exhibit 90–10% rise- and fall-times of about 220 ps under GHz electrical excitation. The data underline the potential of these devices for optical data communication in polymer fibers and free space.

The influence of the growth conditions during capping of CdSe/ZnSe quantum structures grown on GaAs(001) by molecular-beam epitaxy (MBE) were systematically investigated by high-resolution x-ray diffraction, transmission electron microscopy, and temperature dependent, partly time-resolved photoluminescence spectroscopy. The results clearly indicate formation of quantum wells with potential fluctuations if conventional MBE is used for capping the CdSe by ZnSe. In contrast, quantum dot formation occurs using migration enhanced epitaxy for this growth step. In the latter case, quantum dots can be obtained without formation of stacking faults.

The biexciton binding energy EXX is investigated in ZnSe/ZnMgSe quantum wells and quantum wires as a function of the lateral confinement by transient four-wave mixing. In the quantum wells one observes for decreasing well width a significant increase in the relative binding energy, saturating for well widths less than 8 nm. In the quantum wires an increase of 30% is found in the smallest quantum wire structures compared to the corresponding quantum well value. A simple analytical model taking into account the quantum confinement in these low-dimensional systems is used to explain the experimentally observed dependence of the biexciton binding energies.

We have fabricated CdMgTe/CdMnTe based pillar microcavities with diameters down to 2 μm by optical lithography and reactive ion etching. Three-dimensional optical confinement is demonstrated by the occurrence of higher lateral modes. In the high excitation regime, the emission linewidth in pillar microcavities is found to be significantly reduced as compared to planar microcavities.

We demonstrate measurements of the quantum confined Stark effect on single self-assembled CdSe/ZnSe quantum dots. For this purpose, a nano-scaled capacitor was developed being capable of exerting lateral electric fields up to 15 kV / cm on single quantum dots. Stark shifts of up to 1.1 meV have been obtained for the single exciton emission accompanied by a line width broadening due to field-induced carrier tunneling into the barrier. Evaluating the redshift of the luminescence signal as a function of the electric field enables us to extract the polarizability as well as information on the permanent dipole of a single quasi-zero-dimensional exciton.

Time-resolved measurements on self-assembled CdSe/ZnMnSe quantum dots are presented. A rather long excitonic decay time of 580 ps is found which indicates a suppression of nonradiative recombination via the internal Mn 2+ transition. The long excitonic lifetime allows for the formation of magnetic polarons, as evidenced by a transient redshift of the luminescence emission with a time constant of 125 ps. In a magnetic field, a decrease of the measured excitonic decay time is observed and a possible mechanism based on an interplay between bright and dark excitons is discussed.

CdSe/ZnSe quantum structures grown on GaAs(001) by molecular beam epitaxy (MBE) were investigated by temperature dependent, time-resolved photoluminescence spectroscopy. A strong influence of the conditions during cap layer growth on the optical properties was found. Using conventional MBE excitons are localized only at temperatures below 100 K. In contrast, real quantum dot formation occurs using migration enhanced epitaxy for overgrowth of CdSe by ZnSe.

We demonstrate the ability to control the eigenstates in single quantum dots by applying well-defined external fields. Electric fields oriented in-plane as well as perpendicular to the disc-shaped dots allow a modification of the spatial part of the excitonic wavefunction, giving access to the charge distribution in the dot. In contrast, magnetic fields modify the spin part of the wavefunction, resulting in a Zeeman splitting and a diamagnetic shift of the photoluminescence emission. We used the unique property of semimagnetic quantum dots to tailor the effective g-factor, i.e. the sensitivity of the eigenstates to external magnetic fields, by about two orders of magnitude simply by varying the Mn concentration in the dots.

Spatially resolved photoluminescence excitation (PLE) spectroscopy has been performed on both single and single pairs of Cd(Zn)Se/ZnSe quantum dots. While the PLE spectrum in single quantum dots reflects the energy' transfer from excited dot states to the ground state, PLE spectroscopy on a single pair of correlated quantum dots clearly proves the transfer of an exciton between two individual dots.

The chemical vapor deposition (CVD) growth of graphene on copper is controlled by a complex interplay of substrate preparation, substrate temperature, pressure and flow of reactive gases. A large variety of recipes have been suggested in literature, often quite specific to the reactor, which is being used. Here, we report on a relation between growth rate and quality of graphene grown in a scalable 4'' CVD reactor. The growth rate is varied by substrate pre-treatment, chamber pressure, and methane to hydrogen (CH4:H2) ratio, respectively. We found that at lower growth rates graphene grains become hexagonal rather than randomly shaped, which leads to a reduced defect density and a sheet resistance down to 268 Ω/sq.

Single diluted magnetic semiconductor (DMS) quantum dots are studied by means of photoluminescence spectroscopy and magnetoluminescence. The sp-d exchange interaction between a single electron-hole pair and roughly 100 Mn spins within the dot is demonstrated to result in (i) a significant enhancement (more than one order of magnitude) of the emission linewidth and (ii) a strongly modified magnetic field dependence of the polarization degree in a single DMS quantum dot as compared to a non-magnetic reference sample.

Single self-assembled CdSe quantum dots (QDs) are shown to exhibit random fluctuations of the ground state emission energy as well as a spontaneous on/off blinking of the luminescence. A comparison of samples with different densities of extendend crystal defects indicates that Frenkel stacking faults are involved in the mechanism of these phenomena. We demonstrate that single emission lines originating from the same QD show a strong correlation of their transient energy shifts as well as simultaneous blinking.

The Be-enhanced formation of CdSe quantum dots in CdSe/ZnSe heterostructures grown by migration enhanced epitaxy on (001)GaAs substrates has been investigated using photoluminescence spectroscopy, x-ray techniques (diffraction and reflectometry), and transmission electron microscopy. Coverage of the ZnSe starting surface with a fractional monolayer of beryllium selenide leads to enhanced island formation well below the CdSe thickness of 0.6 monolayer corresponding to the onset of the CdSe-rich island formation in the Be-free structures. The effect of the fractional Be coverage is demonstrated by observation of sharp lines in the photoluminescence signal from patterned mesas with dimensions down to 60 nm, which is due to the emission from individual exciton localization sites attributed to quantum dots. X-ray diffraction and reflectometry measurements on CdSe/ZnSe short-period superlattices with the submonolayer CdSe insertions confirm an enhanced roughening of the CdSe layer morphology in the case of bery...

We present photoluminescence (PL) studies on quantum islands formed below the critical thickness for Stranski-Krastanow growth by a well-defined incorporation of Be nucleation centers in CdSe/ZnSe heterostructures. The occurrence of spectrally narrow, individual photoluminescence peaks in spatially resolved PL spectra and a significant enhancement of the recombination lifetime clearly indicate the presence of zero-dimensional states. A direct comparison of temperature-dependent single dot measurements with optical studies on quantum dot (QD) ensembles demonstrates an efficient coupling between individual QD's at elevated temperatures.

Quasi-zero-dimensional magnetic polarons in single Cd0.93Mn0.07Te/Cd0.6Mg0.4Te quantum dots have been studied by photoluminescence spectroscopy. By comparing the experimental data with model calculations, the energy, the internal magnetic field, and the volume of the magnetic polarons are obtained. Moreover, the magnetic environment of the recombining electron hole pair causes a distinct broadening of the emission line (;4 meV) of one diluted magnetic single quantum dot. The alignment of the Mn-spins in high magnetic fields results in a linewidth narrowing of almost one order of magnitude and the linewidth becomes comparable to that of a nonmagnetic Cd0.93Mg0.07Te/Cd0.6Mg0.4Te reference sample. A variety of electronic or magnetic properties of a solid can be artificially tailored by including magnetic ions in a semiconductor crystal matrix. This has been shown, e.g., in diluted magnetic semiconductors ~DMSs!, where the strong sp -d exchange interaction between the charge carriers and the magnetic Mn 21 ions results in a giant Zeeman splitting of the valence- and conduction-band states, a large Faraday rotation, and the formation of magnetic polarons ~MPs!. 1‐3

CdZnSe/ZnSe quantum wires with two different Cd contents and different wire sizes were investigated by resonance Raman spectroscopy. Depending on the Cd content, the longitudinal optical (LO) phonon signal of the wires shows an asymmetry or a clear separation into two peaks. This separation is caused by an inhomogeneous strain parallel to the layer plane due to the strain relaxation at the wire edges. Thus, the observed signals can be attributed to LO phonons excited in the center and in the edge of the structures. Furthermore, high outgoing resonances for the LO overtones (2LO and 3LO) can be observed. For two wire sizes [(a): length L=100 μm, width W=18 nm; (b): L=49 nm, W=27 nm] excitation profiles of the LO fundamental and of the first overtone of Cd0.2Zn0.8Se quantum wires have been recorded. The excitation profiles clearly show structures attributed to the incoming and outgoing resonances. The intensity ratio 2LO/1LO, which is also a good measure for the electron–phonon coupling in a system, was det...

An alternative technological approach for the realization of (Cd,Mn)Te nanostructures is presented. Focused ion beam lithography and subsequent thermal annealing are used for a lateral modulation of the bandgap, allowing both, the realization of quantum wires and quantum dots with adjustable size and a well-defined lateral potential. Due to the high quantum efficiency of the buried nanostructures, photoluminescence spectroscopy even on single quantum dots becomes accessible, allowing an experimental insight on optical transitions in three-dimensionally confined semimagnetic structures.

Room-temperature operation of laterally coupled distributed feedback laser diodes based on ZnSe compounds is presented. Longitudinal single-mode operation with a sidemode suppression of >25 dB is obtained via a gain-coupling mechanism. Within one wafer, the emission energy can be changed over a range of 36 meV in the green spectral range simply by varying the period of the grating. Moreover, Gaussian-shaped near- and far-field patterns are obtained indicating device operation in the fundamental transversal mode.

The influence of exchange interaction on the eigenstate symmetry in a variety o CdSe/ZnSe single quantum dots is studied by photoluminescence spectroscopy. The lifting of spin degeneracy of the optically allowed exciton states results in a linearly polarized doublet of both the single exciton and the biexciton transitions, with an energy splitting reflecting the symmetry of the dots. Comparing different single dots, we found that most of the dots are almost symmetric, i.e. have an energy splitting < 0.2 meV, while strongly asymmetric dots (i.e. a symmetry lower than D 2d ) with energy splittings up to about 1 meV are predominantly oriented along the [110] and [110] axes of the crystal.

We present resonance Raman spectra obtained from CdZnSe/ZnSe quantum wires and short wires with different wire lengths and widths and cadmium contents of 20%. The spectra show high outgoing resonances for the LO overtones (2LO and 3LO). For two wire sizes ((a): length L = 100 μm, width W = 18 nm; (b): L = 49 nm, W = 27 nm) excitation profiles of the LO fundamental and of the first overtone of Cd 0.2 Zn 0.8 Se quantum wires have been recorded. The excitation profiles clearly show structures attributed to the incoming and outgoing resonances. Furthermore, both luminescence intensity and the intensity ratio 2LO/1LO show a decrease with increasing sample age due to aging effects, whereas the phonon wavenumber positions are not affected by the sample age.

This paper presents the results of Raman spectroscopic investigations on etched CdxZn12xSe/ZnSe quantum wires with different wire lengths and wire widths. The recorded spectra show intensive signals in the region of the longitudinal optical phonons (LO) of the ZnSe barrier and the CdZnSe wire as well as several overtones. A strong outgoing resonance can be observed when the overtone position matches the luminescence peak position. The asymmetric LO signal can be separated into three distinct peaks: two higher wavenumber peaks, which are observed especially for the long and thick wires for short excitation wavelengths, are attributed to the LO phonon modes of the ZnSe barrier material and the strained CdZnSe wire; a shoulder at the low wavenumber region, which appears in the spectra of the short and thin wires, is attributed to an LO phonon excited in the strain relaxed wire edges. q 2000 Elsevier Science Ltd. All rights reserved.

Technische Physik, Experimentelle Physik III, Am Hubland, Universita¨t Wu¨rzburg,D-97074 Wu¨rzburg, Germany(Received August 30, 1999)Focused ion beam lithography and subsequent thermal annealing is used to prepare buried CdTe/Cd(Mg,Mn)Te single dots. A well defined lateral potential with a depth of more than 220 meV isachieved, thus exceeding the thermal energy kT at room temperature significantly. The high quan-tum efficiency of the dots allows the investigation of single excitons as well as multiexcitons in thesame single quantum dot.

Cd0.35Zn0.65Se/ZnSe and Cd0.2Zn0.8Se/ZnSe quantum wells grown by molecular beam epitaxy were patterned by electron beam lithography and wet chemical etching to give quantum wires of widths down to the sub-100 nm range. The splitting between the ZnSe LO phonon of the barrier layers and the ZnSe-like LO phonon of the quantum well layer was investigated by applying different excitation wavelengths. For two wire fields with different wire sizes excitation profiles were recorded, which show features belonging to the incoming and outgoing resonance of the LO phonon fundamental vibration and its first overtone. As a result, the LO phonon wavenumber of the wire shows a small shift on changing the excitation wavelength, which is due to changes in the relative intensities of the contributions of the wire center and the wire edges. The wavenumber positions of these two peak contributions are independent of the different resonance conditions at the edge and in the middle of the wire. Copyright © 2000 John Wiley & Sons, Ltd.

We studied the influence of the inhomogeneity of fringe fields originating from nanostructured ferromagnets on the coherent dynamics of localized Mn2+ spins in a (Zn,Cd,Mn)Se/ZnSe quantum well by time-resolved Kerr rotation. By studying hybrid systems with different geometry, we demonstrate that the spatially inhomogeneous fringe field results in two main consequences for the localized spin system: First, a temporal variation of the ensemble precession frequency is obtained, and second, a reduced ensemble spin dephasing time T2* is observed in comparison to an unstructured reference area of the sample.

We demonstrate electrical spin-injection into a single InAs quantum dot embedded in a GaAs-based p–i–n diode structure using a diluted magnetic semiconductor (ZnMnSe) as a spin-source. The spin-information can be extracted directly by evaluating the polarisation degree of the single quantum dot electroluminescence signal. We obtain a spin-polarisation up to 100% at T = 5 K that is decreasing with higher temperature. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In epitaxially grown CdSe quantum dots the LO-phonon coupling is studied by analysing phonon-assisted recombination of a single zero-dimensional exciton. Due to the microscopic disorder of Cd-ions the observed line shape of the LO-phonon replica shows an inhomogeneous broadening and cannot be described by a single Lorentzian peaking at one distinct phonon energy. With increasing excitation density, the radiative decay of biexcitons with a binding energy of 22 meV is observed. Both exciton and corresponding biexciton reveal LO-phonon replica intensities of ∼3% of the respective zero-phonon emission.

We investigated the impact of barrier design on both the photoluminescence and electroluminescence of CdSe quantum dots. Quantum dots embedded in ZnSSe/MgS barriers show a significant improvement of the room temperature quantum efficiency that allows us to detect efficient room temperature emission of a single quantum dot. Embedding the CdSe/ZnSSe/MgS active layer into a p–i–n structure, we could achieve electroluminescence from one single quantum dot at 4 K.

A low damage in situ compatible etching technology for the patterning of wide bandgap II–VI materials has been developed. Thermally assisted ECR-etching in a UHV chamber is used for the pattern transfer into the semiconductor heterostructure while in situ and ex situ photoluminescence (PL) spectroscopy is performed to analyze the etched structures. As one main result, the quantum efficiency in shallow etched near surface quantum wells is reduced after sample exposure to the atmosphere, indicating the creation of non-radiative loss channels at the surface after exposure to air. Moreover, the potential of the UHV etching technology for the fabrication of nanostructures is demonstrated by realizing optically active CdZnSe/ZnSe quantum wires with lateral widths down to 20 nm showing clear lateral confinement effects in the PL spectra.

A systematic experimental and theoretical analysis of the lateral size and composition dependence of strain release and lateral quantization in etched ${\mathrm{Cd}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}$ quantum wires is presented. Wires with lateral structure sizes down to 14 nm were realized by electron beam lithography and wet chemical etching and characterized by photoluminescence (PL) and Raman spectroscopy. For wide wires (${L}_{x}g40 \mathrm{nm}$), the strain relaxation results in a redshift of both the energy of the PL signal and the ${\mathrm{Cd}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{Se}$ LO phonon frequency in the wire region. To model the strain release, theoretical calculations have been performed, minimizing the elasticity energy in the wire cross section. A size-dependent strain release, strongly inhomogeneous across the wire cross section, is obtained. This results in a size- and composition-dependent band-gap shift, which is found to be in good agreement with the experimental data. In narrow wires (${L}_{x}l30 \mathrm{nm}$), the Raman data indicate a saturation of the strain release. As a consequence, clear lateral quantization effects due to the quasi-one-dimensional carrier confinement are observed in the PL spectrum, resulting in a blueshift of the PL signal with decreasing wire width.

The relaxation dynamics of hot excitons was studied in (Zn,Cd)Se/ZnSe quantum wells and quantum dots. A fast population of the radiative excitonic ground state occurs for an excitation excess energy corresponding to an integer number of optical phonon energies. This is indicated by a spectrally narrow photoluminescence peak observed immediately after the exciting laser pulse. Spatial diffusion of excitons, controlled by the interaction between excitons and acoustic phonons, causes a distinct linewidth broadening with increasing delay time in quantum wells. In contrast, this process is found to be strongly suppressed in quantum dots.

Monolayer fluctuations in ultrathin, coherently strained CdS/ZnS quantum structures result in a very strong localization of excitons. The deepest localized excitons can be considered as individual, decoupled and three-dimensionally confined. Consequently, fingerprints of zero-dimensionality are found in the optical spectra like single, ultranarrow luminescence lines in micro-photoluminescence and spectrally broad optical gain in the deep blue spectral range. The exchange splitting is proven and a strong enhancement over the bulk value is observed.

The dimensionality dependence of hot exciton relaxation has been studied by means of time-resolved photoluminescence and photoluminescence excitation spectroscopy comparing CdZnSe/ZnSe quantum wells and quantum dots. An energetically narrow exciton distribution within the inhomogeneously broadened luminescence band is prepared by low density laser excitation with an excess energy corresponding to an integer number of LO phonons. In quantum wells, lateral migration of excitons within the layer causes a transient redistribution of excitons in energy controlled by acoustic phonon interaction. This is indicated by a spectral broadening and a red shift of the PL signal with time. In contrast, this process is found to be substantially suppressed in quantum dots due to the three-dimensional exciton confinement.

Monolayer fluctuations in ultrathin, coherently strained CdS/ZnS quantum-well structures result in a very strong localization of excitons. The deepest localized excitons can be considered as individual, decoupled and three-dimensionally confined. Consequently, the optical properties can be well explained by transitions from an ensemble of spatially distributed, quasi-zero-dimensional excitonic states. The efficient photoluminescence (PL) and the optical gain in the deep-blue spectral range exhibit unusually broad and unstructured spectra. In μ-PL measurements of a mesa-etched SQW sample (10 μm × 10 μm mesas down to 100 nm × 100 nm mesas) single, ultranarrow luminescence lines can be resolved when the number of luminescing states is reduced.

Self-organized InAs and InGaAs quantum dashes have been investigated by chemically sensitive scanning transmission electron microscopy and photoluminescence spectroscopy. The quantum dashes exhibit a triangular cross section. Using electron energy loss spectroscopy, we show that no intermixing between quantum dashes and embedding barrier occurs during growth. By adjusting the nominal thickness of the InAs layer, the emission wavelength of the InAs quantum dashes can be tuned between 1.37 and 1.9 μm in a straightforward way.